Help. ​

y -100x  = 200

Ih he sells no computers, x  =0  .....so....

y  - 100 (0)  = 2000

y  = 2000

3x  - 3y   = 15          divide through by 3

x  - y  = 5             add y to both sides  and subtract 5 from both sides

x - 5  = y

Associative property

Yep...."A"

Note that "C" isn't correct....if it was, we would have that

y  = 5 - x      add x to  both sides

y + x  = 5        rearrange

x + y  = 5        multiply through by 3

3x + 3y  = 15     and this isn't equivalent to  3x  -3y  = 15